8268771: javadoc -notimestamp option does not work on index.html

Reviewed-by: clanger

src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/formats/html/IndexRedirectWriter.java

@@ -77,7 +77,7 @@ private void generateIndexFile() throws DocFileIOException {
         DocType htmlDocType = DocType.forVersion(configuration.htmlVersion);
         Content htmlComment = contents.newPage;
         Head head = new Head(path, configuration.htmlVersion, configuration.docletVersion)
-                .setTimestamp(true, false)
+                .setTimestamp(!configuration.notimestamp, false)
                 .addDefaultScript(false);
 
         String title = (configuration.windowtitle.length() > 0)